Radiation Thermometer, usually referred to as infrared thermometer, is a high-precision non-contact temperature detector, which receives thermal radiation energy of the object to be measured through optical system, converts it into electrical signals, processes data by microcomputer, and displays temperature value on the displayer. Signal processing of the microcomputer inside the radiation thermometer is based on the functional relationship between thermal radiation energy received by instrumentation and temperature of the object to be measured.
In domestic and international prior arts, radiation thermometers are designed on the basis of thermal radiation rule of ideal black body model, where the object to be measured is assumed as ideal black body. The ideal black body is represented as standard blackbody, which is listed in the compulsory verification instrumentation catalogs by metrology laws globally. However, people are facing a problem that objects to be measured are featured by various thermal radiation conditions. A real result will not be available unless the relationship between the thermal radiation rule of ideal blackbody and that of various objects is obtained when radiation thermometer is applied. However, blackbody radiation theory, established at the end of 19th century, in which the difference between ideal blackbody and objects, based on Kirchhoff's Law in classical theories, was simplified as only the radiance. Therefore, it is difficult to correct radiance in a long term when people attempt to establish the relationship between thermal radiation rule of ideal blackbody and that of objects to be measured. The accuracy of temperature measurement can not be improved. Actually, it is one of the difficulties encountered by classical theories. The formula and method applied in prior arts are based on principles of:
I. Principle Using Physical Model of Ideal Blackbody
As an idealized physical model, the ideal blackbody absorbs full incoming radiation and represents maximum radiance. The spectral radiance energy is described with Plank Formula as:E0(λ·T)=C1λ−5(eC2/λT−1)−1  {circle around (1)}
Where E0(λ·T) is spectral radiant flux density of blackbody emission with unit as Wcm−2·m−1; C1, the first radiation constant, is equal to 3.74×10−12 W·cm−2; C2, the second radiation constant, is equal to 1.44 cm·K; λ is the wave length of spectrum radiation with unit as μm; T is the ideal temperature of blackbody with unit as K.
The above is the standard physical model of ideal blackbody. The existence in the nature (objects to be measured), however, has lower absorption and radiation capability than ideal blackbody (referred to as grey body). In order to correct the error between ideal blackbody and grey body, a physical model similar to practice is designed. The spectrum radiance energy of the grey body is described as:E(λ·T)=ε(λ·T)E0(λ·T)  {circle around (2)}
Where ε(λ·T) is the radiance of the object to be measured at Temperature T with radiation wave length λ; 0<ε(λ·T)<1
Formula {circle around (2)} represents that radiation thermometer can be designed on the basis of thermal radiation rule of blackbody, assuming that thermal radiation received by optical system is proportional to E0(λ·T). ε(λ·T) is refined to improve precision of the measurement. However, the thermal radiation received by radiation thermometer is proportional to E(λ·T). Therefore, ε(λ·T) of the object must be obtained in application, which means that radiance correction is required. However, radiance ε(λ·T), which depends on material, surface state, wave length, temperature, radiation condition and environmental factors, cannot be described with explicit formula. The fact that the value of ε(λ·T) cannot be precisely determined is exactly the problem of radiance correction when radiation thermometer is applied.
II. Physical Model Adopting Well-Known Microcomputer-Processed Signal in Radiation Thermometer, Consisting of Narrow Band and Broadband1. E0(λ0T)=C1λ0−5e−C2/λ0T  {circle around (3)}E(λ0·T′)=ε(λ0·T′)E0(λ0·T)  {circle around (4)}for radiation thermometer with narrow working band.in Formula {circle around (3)}, E0(λ0·T) is spectral radiant flux density of ideal blackbody emission with unit as Wcm−2·m−1; C1, the first radiation constant, is equal to 3.74×10−12 W·cm−2; C2, the second radiation constant, is equal to 1.44 cm·K; λ0 is working wave length of infrared temperature detector with unit as μm; T is the absolute temperature of blackbody with unit as K; in Formula {circle around (4)}, E(λ0·T′) is spectrum radiance flux density of the object to be measured (grey body) emission with unit as Wcm−2·m−1; T′ is the temperature of the object; ε(λ0·T′) is the radiance of the object at temperature T′ with radiation wave length λ0(0<ε(λ0·T′)<1). The value of ε(λ0·T′) is difficult to be determined and shall be set up by the user through ε button on instruments.2. E0(λ·T)=σT4  {circle around (5)}E(λ0·T′)=ε(λ0·T′)E0(λ·T)  {circle around (6)}for radiation thermometer with broad working band,in Formula {circle around (5)} and {circle around (6)}, E0(λ·T) is unit radiant exitance of ideal blackbody, including the overall power of various wave length, with unit as W/cm2; σ, the Stefan Constant, is equal to 5.67×10−12 W/cm2·K4; T is the temperature of ideal blackbody; E(λ0·T′) is unit radiant exitance of the object (grey body), including the overall power of various wave length, with unit as W/cm2; T′ is the temperature of the object; ε(λ0·T′) is the radiance of the object at temperature T′ with radiation wave length λ0, where 0<ε(λ0·T′)<1; λ0 is the central wavelength of the working band of the instrument. The value of ε(λ0·T′) is difficult to be determined and shall be set up by the user through ε button on the instrument. In the prior arts, radiation thermometers are calibrated by standard blackbody, the temperature of which is controlled by thermocouple thermometer. Therefore, the temperature of blackbody measured by radiation thermometer shall be consistent with the known controlled temperature. The radiation thermometer calibrated as per the above requirement is only applicable to measure the brightness temperature of the object (When the radiation power of the object is equal to that of blackbody with temperature T, T is defined as brightness temperature of the object.) The real temperature of the object is only available when the value of radiance ε is set by the user.
In general, the temperature measured by the method applied to radiation thermometers in prior arts deviates significantly from the real temperature of the object to be measured. The brightness temperature of the object is measured, while the real temperature is difficult to be determined.